How to forbid/ block Android from connecting to a certain phone provider

I need a way to block my phone from connecting to AT&T since my service provider in Canada only has a roaming plan in the US that uses T-mobile's network. I can force my phone to lock onto T-mobile while I'm roaming in the US, but then I need to change this back to my Canadian provider when i cross back into Canada. I cross daily, so this is a royal PIA. It's a 50/50 chance which provider it decides to connect to when I cross into the US. Would be great if a developer could write a script that allows the user to exclude certain networks to be connected to in Automatic mode.